Acute vascular diseases, such as myocardial infarction, stroke, pulmonary embolism, deep vein thrombosis, peripheral arterial occlusion, and other blood system thromboses are major health risks. They are caused by either a partial, or total, occlusion of a blood vessel by a blood clot.
Accordingly, attempts have been, and continue to be made, to treat these diseases by the use of thrombolytic agents. Such agents dissolve the blood clot thereby removing it from the blood vessel and unblocking the flow of blood.
Because blood clots consist of fibrin, various attempts have been made to dissolve them using plasminogen activators, which are compounds that convert plasminogen, a blood plasma protein precursor, to plasmin, a proteolytic enzyme that degrades the fibrin network of a blood clot to form soluble products [D. Collen, Thromb. Haemostasis, 43, pp. 77-89 (1980); B. Wiman & D. Collen, Nature, 272, pp. 549-50 (1978)].
There are several compounds now available that may function as plasminogen activators in such treatments. These include: streptokinase, a bacterial protein, urokinase, a serine protease isolated from human urine, and two plasminogen activators, tissue-type plasminogen activator and urokinase-type plasminogen activator, that may be extracted from tissues and are produced in certain cells [E. Reich in Proteases And Biological Control (eds. E. Reich et al.), pp. 333-41 (Cold Spring Harbor Laboratory, New York) (1975)].* FNT * In this application the plasminogen activator nomenclature proposed at the XXVIII Meeting of the International Committee on Thrombosis and Hemostasis, Bergamo, Italy (July 27, 1983) will be used: tissue-type plasminogen activator ("t-PA") and urokinase-type plasminogen activator ("u-PA").
Each of these plasminogen activators has certain limitations. For example, streptokinase and urokinase do not have a high affinity for fibrin. As a result, they may convert both circulating and fibrin-bound plasminogen to plasmin. This may be a significant disadvantage, because not only is the plasmin formed in the circulating blood of marginal utility in dissolving blood clots, but it also may contribute to hemorrhagic potential by degrading various clotting factors, e.g., fibrinogen and Factor VIII. Urokinase-type plasminogen activator has similar disadvantages. Accordingly, neither streptokinase nor urokinase therapy is likely to be fully satisfactory in the treatment of vascular disease.
Tissue-type plasminogen activator ("t-PA"), on the other hand, has a higher affinity for fibrin than urokinase-type plasminogen activator [O. Matsuo et al., Thromb. Haemostasis, 45, pp. 225-29 (1981); C. Korninger et al., Thromb. Haemostasis, 46, pp. 561-65, 658-61, 662-65, (1981); M. Hoylaerts et al., J. Biol. Chem., 257, pp. 2912-29 (1982)]. Moreover, some studies suggest that t-PA may selectively seek out and dissolve blood clots without the potential hemorrhagic side effects associated with other plasminogens and treatments based on them [FDC Reports, T & G-3-4 (Mar. 26, 1984)].
Therefore, attempts have been made to produce and to evaluate clinically human t-PA in thrombolytic agents and therapy. Such attempts are referred to, for example, in European patent application Nos. 93619 and 99126. Application No. 99126 refers to a thrombolytic composition said to contain a t-PA isolated from human kidneys and bood vessels. Application No. 93619 refers to human t-PA said to be produced through recombinant techniques. See also D. Pennica et al., "Coning And Expression Of Human Tissue-Type Plasminogen Activalor cDNA In E.coli", Nature, 301, pp. 214-221 (1983).
However, these native and recombinant t-PAs may also not be fully satisfactory in on-going therapy to dissolve blood clots, because they may be cleared from the blood faster than is desirable for optimum utility and effectiveness in therapy. For example, the half-life of native t-PA is about 3 minutes; the t-PA being cleared from the blood by a single passage through the liver. Therefore, very high, continuous dosage therapy must be employed. For example, it has been reported that doses of 80-100 mg of recombinant t-PA are now required. Therefore, plasminogen activators that are highly selective for fibrin, and also have extended lifetimes in the blood, would be more satisfactory and potentially more useful in thrombolytic agents and therapy.